Apparatus for measuring characteristics of a liquid

ABSTRACT

An apparatus for measuring characteristics of a liquid including a channel through which, during operation, the liquid flows and a plurality of sensors for measuring characteristics of the liquid as it flows through the channel. The cross-sectional shape of the channel varies along its length, but the cross-sectional area of the channel is substantially constant so that, during operation, liquid flows through the channel at a substantially uniform speed. The sensors include an ultra-violet sensor and an infra-red sensor, each having a source and a detector of electromagnetic radiation arranged along an axis transverse to the channel. The axes of the two sensors are transverse to each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an apparatus for measuring characteristics ofa liquid. In particular, but not exclusively, the invention relates toan ultraviolet (UV) absorbency-based monitor for measuring the amount oforganic pollution in a liquid, and is of application for on-linemonitoring of water quality at locations such as industrial or watertreatment plant effluent outlets, rivers or reservoirs.

2. Description of the Prior Art

A number of methods are available for determining the biological ororganic pollution in water, involving the measurement of parameters suchas BOD (biochemical oxygen demand), TOC (total organic carbon) or COD(chemical oxygen demand). The parameter that is usually of most interestis the BOD.

A number of on-line, continuous BOD monitors are available that operateby measuring the absorbency of ultraviolet (UV) light by a sample. Thesemonitors are based on the principle that a particular substance willabsorb light of a particular wavelength (or wavelengths), so that thereduction in intensity of light of that wavelength, when transmittedthrough a sample containing the substance, can be related to theconcentration of that substance. Aromatic organic compounds, and otherorganic compounds that have conjugated double bonds, absorb light in theUV wavelength region and research has shown that absorbency measurementstaken in that region, and in particular at 254 nm, can be related to BOD(and also to TOC and COD).

In one such monitor, the amount of organic matter in a liquid sample isdetermined by passing a liquid sample through a flow cell, passing UVand visible light through the sample and making measurements of theabsorption of UV light by the sample and the absorption and scatter ofvisible light. The amount of organic matter in the sample is determinedfrom the measured absorption of UV light, with the visible lightmeasurements being used for compensation purposes.

One disadvantage of the known monitor is the tendency of the flow cellto become fouled by substances in the liquid sample, which can result inthe windows for the UV and visible light monitors becoming obscured.This reduces the accuracy and efficiency of the monitor and caneventually prevent operation. It is necessary, therefore, to clean theflow cell at regular intervals, which is costly and time-consuming andcan prevent automatic operation of the monitor.

OBJECTS AND SUMMARY OF THE INVENTION

There is a need, therefore, for an apparatus for measuringcharacteristics of a liquid, that resists fouling by substances in theliquid and which can clean itself when fouling occurs.

According to the present invention there is provided an apparatus formeasuring characteristics of a liquid, the apparatus including a channelthrough which, during operation, the liquid flows and a plurality ofsensors for measuring characteristics of the liquid as it flows throughthe channel, the cross-sectional area of the channel being substantiallyconstant so that, during operation, liquid flows through the channel ata substantially uniform speed.

This reduces turbulence in the liquid, which in turn helps to reducefouling of the channel by substances in the liquid.

Advantageously, the cross-sectional shape of the channel varies alongits length, according to the requirements of the different sensors. Thecross-sectional shape of the channel may vary to accommodate one or moresensors located within the channel. By varying the shape of the channelwhile keeping its cross-sectional area constant, the needs of thedifferent sensors can be met, without causing turbulence in the liquid.

Advantageously, the two transverse axes of the channel are of differentlengths, at least one sensor being arranged to measure a characteristicof the liquid between points at opposite ends of one of the transverseaxes and at least one other sensor being arranged to measure acharacteristic of the liquid between points at opposite ends of theother transverse axis.

Advantageously, at least two of the sensors are optical sensors, theoptical path length through the liquid of a first of the optical sensorsbeing longer than the optical path length through the liquid of thesecond optical sensor. Preferably, each optical sensor comprises asource of electromagnetic radiation located on one side of the channeland a detector of electromagnetic radiation located on the opposite sideof the channel.

The sensor arrangement may be substantially the same as that describedin GB 2256043A, the description of which is incorporated herein byreference.

Advantageously, the channel is shaped to produce a streamline flow ofliquid through the channel.

The present invention further provides an apparatus for measuringcharacteristics of a liquid, the apparatus including a channel throughwhich the liquid flows, a plurality of sensors for measuringcharacteristics of the liquid as it flows through the channel, and adevice for washing the channel. The provision of a washing device allowsthe channel to be cleaned automatically to prevent excessive depositsbuilding up in the channel.

The washing device may include a spray head that moves along the channelduring operation of the washing device. The channel isadvantageously-stepped, so that the spray head moves along one region ofthe channel in a first part thereof, and along a second region of thechannel in a second part thereof. The spray head may include outlets todirect the washing liquid substantially perpendicularly to itslongitudinal axis. The outlets may be arranged around substantially theentire circumference of the spray head.

The spray head may be driven hydraulically along the channel.Advantageously, the spray head is hydraulically driven by means of thewashing liquid.

Advantageously, the spray head is formed on the end of a hollow rod, therod is mounted for sliding movement along a hydraulic cylinder and thehydraulic cylinder has an inlet opening for washing liquid, thearrangement being such that when washing liquid is supplied underpressure to the hydraulic cylinder, the hollow rod is driven along thecylinder and washing liquid flows through the hollow rod to the sprayhead. The rod may have one or more radial slots formed its end, theslots forming the liquid outlets of the spray head.

The hydraulic cylinder may include means for returning the spray head toits starting position. The returning means may include a spring and/ormeans for driving the spray head hydraulically back to its startingposition.

The apparatus may include means for storing a supply of washing liquidand/or means for heating the washing liquid. The apparatus may includemeans for supplying a chemical cleaning agent to the channel.

The apparatus may include an inlet filter, located upstream of the inletchamber, for removing large bodies from the liquid sample.

The apparatus may include means for automatically controlling theoperating sequence of the apparatus. The control means advantageouslycontrols operation of the apparatus according to the following operatingsequence:

(a) a liquid sample is pumped through the channel and itscharacteristics are determined by the sensors;

(b) the liquid sample is drained from the channel;

(c) the washing device is activated to wash the channel;

(d) the channel is flushed with clean water and then drained;

(e) the channel is washed with a chemical cleaning agent and thendrained, and

(f) the inlet filter is back-washed with clean water.

The apparatus may be for determining the amount of organic matter in aliquid. Advantageously, the apparatus includes first means for passinglight of a wavelength in the UV region through the liquid sample andsensing the emergent UV light intensity, second means for making ameasurement to provide an indication of the absorption by the sample oflight, and third means for making a further measurement to provide anindication of the amount of scatter of light caused by the sample.

The apparatus may include processing means for determining the amount oforganic matter in the sample from an output of the first means, adjustedin accordance with outputs from the second and third means.

The present invention yet further provides, a method of cleaning achannel which is in an apparatus for measuring characteristics of aliquid and through which the liquid flows, the method comprising thesteps of draining the liquid from the channel and moving a washingdevice having a spray head along the channel whilst spraying the channelwith washing liquid.

Advantageously, the method further comprises the step of flushing thechannel with washing liquid.

Advantageously, the method further comprises the step of flushing thechannel with a chemical cleaning agent, for example a biocide or adetergent.

The method may further comprise the step of back-washing an inlet filterwith washing liquid.

Advantageously, the operating sequence of the steps is controlledautomatically.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example, an embodiment of the invention will now be describedwith reference to the accompanying drawings, in which:

FIG. 1 is a front view of the hydraulic system of the apparatus;

FIG. 2 is a diagrammatic representation of the hydraulic system;

FIG. 3 is a perspective view of the flow cell and the washing device;

FIG. 4 is a perspective view front cover of the flow cell;

FIG. 5 is a perspective view of the flow cell with the front coverremoved;

FIG. 6 is a perspective view of the assembled flow cell, showinginternal structural details;

FIGS. 7 and 8 are side views of the flow cell and the washing device incross section;

FIG. 9 is a perspective view of the spray head of the washing device;

FIG. 10 is a cross-section through the flow cell on line X--X;

FIG. 11 is a cross-section through the flow cell on line XI--XI;

FIG. 12 is a bottom view of the inlet device;

FIG. 13 is a side view in cross-section of the inlet device, and

FIG. 14 is a logic diagram, showing the operating sequence for thevalves and pumps of the apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The monitor, which is capable of being used for on-line, continuousmonitoring of liquid quality in a variety of on-site locations,comprises a single, weatherproof unit having separate compartmentscontaining the hydraulic system and the electronic measurement andcontrol circuits of the monitor.

The hydraulic system is shown schematically in FIG. 2 and includes aflow cell 1 having a fluid flow channel 2 with an inlet opening 3 at oneend and an outlet opening 4 at the other end. The flow cell 1 includes anumber of sensors for measuring characteristics of the liquid flowingthrough the channel. Those sensors include a pressure switch 5, atemperature probe 6, an optical sensor 7 comprising one or more infraredor visible light LEDs, probes 8,9 for measuring the pH and conductivityof the liquid sample and a UV sensor 10, which may operate at one ormore UV wavelengths.

A sampling device 12 for drawing a liquid sample from a sample ditch 13is connected via a filter 14 and a valve V7 to a pump PA, and from therevia valve V4 to the inlet opening 3 of the channel 2. The outlet opening4 of the channel 2 is connected via valve V6 and a drain line 15 to anoutlet 16, for returning the sample to the sample ditch 13.

The hydraulic system includes a jet washing device 20 for cleaning thechannel 2 in the flow cell 1. The jet washing device 20, which iscoaxially arranged with respect to the flow cell 1 (as shown in FIG. 7),includes a spray head 21 mounted on the end of a hollow rod 22. Thehollow rod 22 includes a piston 23 at its other end and is mounted forsliding movement in a hydraulic cylinder 24. The bore of the hollow rod22 connects the spray head 21 to the primary chamber 25 of the cylinder24. A return spring 26 is provided in the secondary chamber 27 of thecylinder 24, between the piston 23 and the end wall of the cylinder.

The primary chamber 25 of the cylinder 24 is connected via valves V9,V1, V2, V3 and pump PB to a tank 30 of potable water, which is filledvia valve V8, input filter 31 and one-way valve 32 from a supply 33.Water may be pumped by pump PB from the tank 30 into the primary chamber25, forcing the piston 23 along the cylinder 24 against the resistanceof the return spring 26. This causes the spray head 21 to move along theflow channel 2 of the flow cell 1. At the same time, water flows fromthe primary chamber along the bore in the hollow rod 22 to the sprayhead 21, where it emerges to provide a number of powerful jets forcleaning the channel 2 of the flow cell 1. The water may be heated by aheater H1 in a tank 34, located downstream of the tank 30. If desired,liquids other than potable water may be used for cleaning the channel 2.For example, raw water or effluent may be used.

By activating valve V3, a chemical cleaning agent, for example adetergent or a concentrated biocide, may be pumped from a tank 36 intothe cylinder 24 to provide a jet wash. Valve V6 is also then activated,to return the cleaning agent from the outlet opening 4 of the channel 2to the tank 36.

Outward movement of the piston 23 is damped by water in the secondarychamber 27 of the cylinder, which escapes as the piston advances throughan adjustable needle valve 40 and via valve V9 to the sample ditch 13.By activating valve V9, water may be pumped into the secondary chamberof the cylinder through a one-way valve 41, to drive the piston 23 backto its starting position. Water in the primary chamber 25 escapes as thepiston 23 returns via valve V9 and the outlet 16 to the sample ditch 13.

Activating valve V1 allows potable water (heated if necessary) to bepumped via valve V5 directly into the into inlet opening 3 of thechannel 2. A chemical cleaning agent may be added to the water byactivating a metering pump PC.

The filter 14 may be back-washed by activating valves V2 and V7 and pumpPB, causing potable water to flow through valves V10 and V11 to thesample ditch 13.

Activating valve V10 allows-water to be pumped into the tank 36, to washthe tank. Activating valve V11 and pump PB allows water to be drainedfrom the water tank 30. A line 45 containing an adjustable pressurerelief valve 46 connects water supply line 47 on the upstream side ofvalve V9 to the drain line 15, to allow water to flow to the sampleditch 13 if the water pressure exceeds a predetermined value. A sampletap 48 is provided in the drain line 15 upstream of the outlet 16, toallow a sample of liquid leaving the flow cell 1 to be drawn for testingpurposes.

The flow cell 1 and the jet wash device are shown in more detail inFIGS. 3 to 8. The flow cell 1 comprises a main housing 50 and a frontcover 51 that are joined to one another in a face-to-face relationshipby bolts 52. A groove 53 extends around the periphery of the engagementface 54 of the front cover 51, and receives an O-ring. When the frontcover 51 and the main housing 50 are bolted together, the O-ring iscompressed between the engagement faces 54,55 of the front cover 51 andthe main housing 50, to form a water-tight seal.

A groove 58 of elongate rectangular cross section is formed in theengagement face 55 of the main housing 50. The three faces of the groove58 form the top, bottom and one side wall of the fluid flow channel 2.The groove 58 includes a step 59 of approximately half its height,approximately half way along the groove.

The front cover 51 includes a similarly-shaped raised formation 62 onits engagement face 55, which fits into the groove 58 when the mainhousing 50 and the front cover 51 are joined to one another. The uppersurface 63 of the formation 62 forms the other side wall of the channel2.

Enlarged portions at either end of the groove 58 form the inlet andoutlet chambers 65,66 for the liquid sample. Fluid flow channels 67having fittings for fluid supply lines at their outer ends extend fromthe top, bottom and end faces of the main housing to the inlet andoutlet chambers 65,66.

Two cylindrical bores 70 extend downwards from the top face of the mainhousing 50 to intersect the groove 58 between the inlet chamber 65 andthe step 59. The bores 70 provide insertion channels for theconductivity and pH probes 9,8 (which may also include a temperaturesensor). Recesses 71 are provided in the two side walls 72 of thechannel 2 adjacent the conductivity and pH probes 9,8, widening thechannel at those points (as can be clearly seen in FIGS. 4, 5 and 10).The recesses 71 are shaped such that the cross-sectional area of thechannel 2, with the probes 9,8 inserted, remains constant throughout itslength. This ensures that liquid flows through the channel 2 at aconstant speed, which reduces turbulence and minimizes fouling of thechannel.

Downstream of the bores 70 for the insertion probes 9,8, between thestep 59 and the outlet chamber 66, two further bores 74 extendvertically from the top and bottom faces of the main housing 50, tointersect the top and bottom walls of the channel 2 at opposite points.The bores 74 house an infrared (IR) monitor 75, for assessing thescattering and absorption by the liquid sample. The monitor 75 comprisesan IR light source 76 comprising one or two light emitting diodes (LEDs)77 and a detector unit 80, including one or two corresponding siliconphotodiodes 81. Windows 82 are provided at the inner ends of the lightsource 76 and the detector unit 80. As shown in FIG. 11, the source 76and the detector unit 80 are located at opposite ends of the longer axisof the elongate rectangular cross section channel 2, so that the pathlength of the IR light through the liquid sample is long.

A horizontal bore 85 having a window 86 at its inner end extends throughthe front cover 51 to intersect the light path of the IR light as itpasses through the channel 2. The bore 85 houses a further photodiode87, which measures the amount of light scattered by the sample.

Further downstream, three further sets of bores 90 extend horizontallythrough the main housing 50 and the front cover 51 to intersect the sidewalls 72 of the channel. Windows 91 are provided at the inner ends ofthe bores. The bores 90 house an ultraviolet (UV) monitor 10, comprisinga source 92 of UV and visible light located adjacent the bores on oneside of the flow cell 1 and three detector 93 located in the oppositebores. As illustrated in FIG. 11, the source 92 and the detectors 92,93are positioned at opposite ends of the shorter axis of the elongaterectangular cross-section of the channel 2, so that the path length oflight through the liquid sample is short. The three monitors 10 operateat different wavelengths, for example at ultraviolet wavelengths 254 nmand 313 nm and the visible wavelength 405 nm. The visible band at 405 nmis used for determining the color of the sample.

A further bore 95 extends from the inlet chamber 65 parallel to thelongitudinal axis of the flow cell 1 to the adjacent end face 96 of themain housing 50. A fitting 97 for the jet wash device 20 is provided atthe outer end of the bore. The bore 95 is positioned at the lower end ofthe inlet chamber 65 so that, when the jet wash probe extends into thechannel 2, as shown in FIGS. 7, 8, 10 and 11, the spray head 21 movesalong the bottom of the channel upstream of the step 59, so that itpasses underneath the conductivity and pH probes 9,8 and along thecenter of the channel 2 downstream of the step 59, so that it cleans theUV detector windows 91 effectively.

As shown in FIG. 9, the spray head 21 is formed by cutting four radialslots 98 in the cylindrical wall of the hollow rod 22 and closing theend of the rod. The slots 98 are equiangularly displaced around thecircumference of the rod 22 and each extends over an arc of at least90°, so that the spray head 21 provides a full 360° jet wash.

The sampling device 100 is shown in more detail in FIGS. 12 and 13. Thesampling device 100 includes a square top plate 101 having a centralaperture 102 to which an inlet line is connected. A square float 103having a hollow center is attached to the lower surface of the top plate101 and a filter mesh 104 extends across the opening in its lower face.When the sampling device is placed in the sample ditch, it floats withthe filter mesh 104 just beneath the surface of the liquid, to preventthe mesh becoming obstructed by floating matter. When pump PA isactivated, a sample is drawn into the inlet line through the filter mesh104.

The usual operating sequence of the monitor is as described below, thevalves and pumps activated in each step of the operating sequence beingrepresented diagrammatically in FIG. 14. The operating sequence isnormally carried out automatically under the control of amicroprocessor.

During normal operation of the monitor, all the valves are deactivatedand only pump PA is activated to draw a sample of liquid from the sampleditch and pump it at a steady rate through the flow cell 1 for testing.The smooth shape of the channel in the cell and its uniform crosssectional area ensure that the flow of liquid through the channel issubstantially streamline (i.e. without turbulence), which helps toprevent fouling of the cell.

Eventually, however, the cell will need to be cleaned, and the firststep in the cleaning operation is to allow the sample liquid to drainfrom the flow cell 1 by deactivating the pump PA. Valves V4 and V5 andpump PB are then activated to wash the channel with potable water, usingthe jet wash device 20. When the spray head reaches the end of thechannel 2, valve V9 is activated and pump PB is driven at half power toreturn the spray head to its starting position.

The channel is then flushed with water by activating valves V1 and V11and operating pump PB at half power, after which it is drained.

The next step is a wash with a chemical cleaning agent, which isachieved by activating valves V1, V3, V4 and V8 and operating pump PB athalf power. The cleaning agent is then allowed to drain from the channelto the tank 36 by deactivating the pump PB.

Finally, the filter 14 is back washed with water by operating valves V2and V7 and the pump PB.

Other operating steps, which may form part of the operating sequence ormay be for maintenance of the monitor, include flushing the channel 2with a mixture of water and cleaning agent, providing a jet wash withhot water, filling the water tank with water, emptying the cleaningagent tank to a service container, draining the cleaning agent tank tothe sample ditch and filling the tank from a service container. Thevalves and pumps activated for each of those steps is representeddiagrammatically in FIG. 14.

Although the invention has been described primarily in terms of anorganic pollutant monitor, it may also be used for other applications.

What is claimed is:
 1. An apparatus for measuring characteristics of aliquid, the apparatus including a channel through which, duringoperation, the liquid flows and a plurality of sensors for measuringcharacteristics of the liquid as it flows through the channel, theinternal cross-sectional shape of the channel varying to accommodate aplurality of sensors located within the channel for measuring respectivecharacteristics of the liquid, the internal cross-sectional area of thechannel being substantially constant so that, during operation, liquidflows through the channel at a substantially uniform speed, the sensorsincluding a first optical sensor which comprises a source ofelectromagnetic radiation and a detector of electromagnetic radiationand which is arranged along a first axis transverse to the channel and asecond optical sensor which comprises a source of electromagneticradiation and a detector of electromagnetic radiation and which isarranged along a second axis transverse to the channel and transverse tothe first axis, the optical path length through the liquid of the firstoptical sensor being longer than the optical path length through theliquid of the second optical sensor.
 2. An apparatus according to claim1, in which the wavelength of the electromagnetic radiation detected bythe detector of the first sensor is in the infra-red region and thewavelength of the electromagnetic radiation detected by the-detector ofthe second sensor is in the ultra-violet region.
 3. An apparatusaccording to claim 1, in which the channel is shaped to produce astreamline flow of liquid through the channel.
 4. An apparatus accordingto claim 1 including a device for washing the channel.
 5. An apparatusaccording to claim 4, in which the washing device includes a spray headthat moves along the channel during operation of the washing device. 6.An apparatus according to claim 5, in which the channel is stepped, sothat the spray head moves along one region of the channel in a firstpart thereof, and along a second region of the channel in a second partthereof.
 7. An apparatus according to claim 5, in which the apparatusincludes a supply of washing liquid and the spray head includes outletsto direct the washing liquid substantially perpendicularly to itslongitudinal axis.
 8. An apparatus according to claim 7, in which theoutlets are arranged around substantially the entire circumference ofthe spray head.
 9. An apparatus according to claim 5, in which the sprayhead is driven hydraulically along the channel.
 10. An apparatusaccording to claim 9, in which the apparatus includes a supply ofwashing liquid and the spray head is hydraulically driven by means ofthe washing liquid.
 11. An apparatus according to claim 5, in which thespray head is formed on the end of a hollow rod, the rod is mounted forsliding movement along a hydraulic cylinder and the hydraulic cylinderhas an inlet opening for washing liquid, the arrangement being such thatwhen washing liquid is supplied under pressure to the hydrauliccylinder, the hollow rod is driven along the cylinder and washing liquidflows through the hollow rod to the spray head.
 12. An apparatusaccording to claim 11, in which the rod has one or more radial slotsformed at its end, the slots forming the liquid outlets of the sprayhead.
 13. An apparatus according to claim 11, in which the hydrauliccylinder includes means for returning the spray head to its startingposition.
 14. An apparatus according to claim 13, in which the returningmeans includes a spring.
 15. An apparatus according to claim 13, inwhich the returning means includes means for driving the spray headhydraulically back to its starting position.
 16. An apparatus accordingto claim 4, including means for storing a supply of washing liquid. 17.An apparatus according to claim 4, including a supply of washing liquidand means for heating the washing liquid.
 18. An apparatus according toclaim 4, including means for supplying a chemical cleaning agent to thechannel.
 19. An apparatus according to claim 4, including an inletchamber upstream of the channel and an inlet filter, located upstream ofthe inlet chamber, for removing large bodies from the liquid.
 20. Anapparatus according to claim 19, in which a control means controls theoperation of the apparatus according to the following operatingsequence:(a) a liquid sample is pumped through the channel and itscharacteristics are determined by the sensors; (b) the liquid sample isdrained from the channel; (c) the washing device is activated to washthe channel; (d) the channel is flushed with clean water and thendrained; (e) the channel is washed with a chemical cleaning agent andthen drained, and (f) the inlet filter is back-washed with clean water.21. An apparatus according to claim 4, including means for automaticallycontrolling the operating sequence of the apparatus.
 22. An apparatusaccording to claim 1, for determining the amount of organic matter in aliquid.
 23. An apparatus according to claim 22, the apparatus being soarranged that, during operation,(a) light of a first wavelength, in theUV region, is passed through the liquid sample so that a firstmeasurement may be made that provides an indication of the emergent UVlight intensity, (b) light of a second wavelength, different from thefirst wavelength, is passed through the liquid sample so that a secondmeasurement may be made that provides an indication of the absorption bythe liquid sample of light of that second wavelength, and (c) a thirdmeasurement may be made that provides an indication of scatter of lightcaused by the liquid sample.
 24. An apparatus according to claim 23,including processing means for determining the amount of organic matterin the liquid sample from an output representative of the firstmeasurement, adjusted in accordance with outputs representative of thesecond and third measurements.